Εκτίμηση της περιεκτικότητας επιφανειακού αιωρούμενου υλικού (SPM) στον εξωτερικό Θερμαϊκό Κόλπο, με την χρήση δορυφορικών εικόνων Landsat 8

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1 Εκτίμηση της περιεκτικότητας επιφανειακού αιωρούμενου υλικού (SPM) στον εξωτερικό Θερμαϊκό Κόλπο, με την χρήση δορυφορικών εικόνων Landsat 8 Τσάπανου Α. 1, Οικονόμου Ε. 2, Σκιάνης Γ. 1 και Πούλος Σ. 1 1 Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών, atsapanou@geol.uoa.gr, poulos@geol.uoa.gr 2 Τμήμα Πολιτικών Μηχ.ΤΕ και Μηχ. Τοπογραφίας & Γεωπληροφορικής ΤΕ, T.E.I. Αθήνας, eoikonomou@teiath.gr Περίληψη Στην παρούσα εργασία γίνεται επεξεργασία δορυφορικών εικόνων Landsat 8 για την εκτίμηση της επιφανειακής περιεκτικότητας αιωρούμενων ιζημάτων (SPM) στην ευρύτερη περιοχή του Θερμαϊκού Κόλπου. Γενικά παρατηρούνται χαμηλές συγκεντρώσεις αιωρούμενου υλικού με μέγιστες τιμές στα στόμια ποτάμιων απορροών, γεγονός που χαρακτηρίζει ως διαυγή τα νερά και προσελκύει το τουριστικό ενδιαφέρον κατά μήκος των ακτών του. Στην παρούσα μελέτη εξετάζονται δύο αντιπροσωπευτικές δορυφορικές εικόνες Landsat 8 κατά την υγρή (Νοέμβριος-Απρίλιος) και ξηρά (Μάιος-Οκτώβριος) περίοδο. Στα νερά του εξωτερικού Θερμαϊκού Κόλπου εμφανίζεται φτωχή περιεκτικότητα SPM (0-2.5 mg/l), χωρίς εποχιακή διακύμανση, η οποία συσχετίζεται με τις ποτάμιες απορροές. Στα σημεία εκβολών παρατηρούνται οι υψηλότερες συγκεντρώσεις (>1 mg/l). Εμφανής είναι η ύπαρξη ενός κύριου κυκλώνα στα επιφανειακά νερά του εξωτερικού Θερμαϊκού που δημιουργείται τοπικά κατά τον μήνα Αύγουστο του Λέξεις κλειδιά: διαύγεια υδάτων, τηλεπισκόπηση, ΒΔ Αιγαίο Πέλαγος Estimation of surface suspended particulate matter concentration (SPM) in the outer Thermaikos Gulf, by using Landsat 8 data Tsapanou, A. 1, Oikonomou, E. 2, Skianis, G. 1 and Poulos, S. 1 1 Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, atsapanou@geol.uoa.gr, poulos@geol.uoa.gr 2 Department of Civil Engineering and Surveying & Geoinformation, T.E.I. of Athens, eoikonomou@teiath.gr Abstract In the present work satellite data from Landsat 8 are processed in order to estimate the surface suspended particulate matter (SPM), within the Thermaikos Gulf, northern Greece. In general, low SPM concentrations characterize the area with the highest values to appear in front of the river mouths. This attracts the tourist interest, because of the water clarity. In this study we investigate the SPM values by examining two representative Landsat 8 satellite images, during the wet (November- April) and the dry (May-October) season. In the outer Thermaikos waters the SPM concentrations are low (0-2.5 mg/l) without any seasonal fluctuation and they are correlated with the river outlets. The highest values appear in front of the river mouths (>1 mg/l). Particularly, on 14th of August 2014 a cyclonic circulation over the surface waters of the central outer Thermaikos Gulf is clearly visible. Keywords: water transparency, remote sensing, NW Aegean Sea

2 1. Introduction 11th Panhellenic Symposium of Oceanography and Fisheries The entire eastern Mediterranean Sea shows poor content of suspended particulate matter (Karageorgis et al., 2008). Thermaikos Gulf belongs to this marine system and displays the same features, presenting the larger contents of suspended material close to river mouths that flow into the Gulf (e.g.> 40 mg / l at the mouth of Pinios river (Lazogiannis et al., 2014 )), during the wet periods (November-April) and mainly in surface waters (Karageorgis et al., 2008). The total amount of sediment inflow to the Gulf from the river supplies is calculated at 35.4x109 tones in the upper Quaternary (Lykousis et al., 2005). The coastal zone of the outer Thermaikos Gulf and particularly the coastal areas along the west coast are characterized by great touristic development, which exists partly due to the natural beauties of the area (e. g. N. Passengers, Ag. Triada Epanomi, Angelochori beaches) and also the clarity of the water. The water in general, show little turbidity, but it is influenced by river discharges and resuspension processes (Zervakis et al. 2005). Corresponding previous estimations of the suspended particulate material dispersion in Thermaikos region has been made by Karageorgis et al., (2000), correlating satellite images and in situ data. The concentration -plume of Pinios river- was not visible in panchromatic Landsat 5 image used, while in field measurements its existence was evident. Until recently, the calculation of the suspended material content (suspended particulate matter concentration-spm) was estimated from satellite data using receivers such as the Aqua / MODIS, Orbview-2 / SeaWiFS etc. exploiting the marine reflection at a single red channel. The purpose of this paper is a first attempt to retrieve the surface suspended particulate matter concentration in the waters of the outer Thermaikos Gulf by using satellite images, and then to take a more extensive study in the estuary region (Pinios delta-axios-aliakmon), by comparing field data. 2. Materials and Methods The selected Landsat 8 images of the region of interest have been taken from the file provided by the United States Geological Survey (USGS) Earth Resources Observation and Sciences (EROS) Centre. The access is free via the Internet. The system of the satellite Landsat 8 (L8) consists of two sensors; the OLI (Operational Land Imager) 8 multispectral channels (4 channels in the visible and 4 to near infrared) with a spatial resolution of 30 m. and a panchromatic channel with spatial resolution 15 m., and the sensor TIRS (Thermal InfraRed Sensor) with two channels in thermal infrared of spatial resolution 100 m. In this paper two images from 2014 have been analyzed, one during the wet and one during the dry season, with imageries taken on 03/23/2014 and 14/08/2014 and cloud cover 32% and 19%, respectively. The images were processed using the software ENVI 5.1 and held atmospheric correction to channel 4 (red) and 5 (near infrared), based on the methodology of Vanhellemont et al. (2014). Images were corrected for effects of molecular scattering (Rayleigh) and aerosols (aerosols), to recover the water reflectance-pw (water-leaving radiance reflectance) at the red channel. The content of suspended sediment (SPM) on channels 4 ( nm) and 5 ( nm) has been resulting after calculating the pw values, using the algorithm for the single channel of OLI 4 to 650 nm (Nechad et al., 2010).

3 3. Results During March 2014 the values of suspended particulate matter presented a range from 0 to 2.5 mg/l. The largest concentrations are found near the estuary, on the east coast of Thessaly and at the inner Thermaikos Gulf. Characteristically, at the mouth of Pinios river the suspended material has values between mg/l, while in the central part of the outer Thermaikos the values show a declination ( mg/l) (Fig. 1). The suspended material transport creates a flow from the mouth of Pinios River towards the south, possibly due to the Coriolis force. Aliakmon, Axios & Pinio (a) (b) (c) Figure 1. (a) Unprocessed Landsat 8 image in March 2014, (b) the same image after process with atmospheric correction and SPM content (mg/l) and (c) enlarge the area of Pinios river estuary with the SPM values (mg/l). Figure 2 (b, c) shows the study area in 14 th of August After processing it is observed that the SPM values range from 0-2 mg/l. The maximum concentrations are found close to river mouths of that discharge into the sea. Especially, off the Pinios delta a progressive increase in the SPM is shown, from the river mouth to the northwestern coast. Values of SPM in the outer Thermaikos Gulf are smaller and are minimized eastwards, i.e. the coast of Chalkidiki, which demonstrates the clarity of water and justifies the great development of tourism on the shores of the outer Thermaikos Gulf. Previous studies within the region (Karageorgis et al., 2001) have identified cyclonic circulation patterns to explain the concentrations of SPM. Their existence in the central part of the outer bay and eastwards the Pinios River estuary is also evident here (Fig. 2b). This cyclone is expected to have little impact on concentrations of particulate matter, as during this period (summer) there are not significant terrestrial inflows and its presence is likely due to the Etesian.

4 L Pin (a) (b) (c) Figure 2. (a) Unprocessed Landsat 8 image in 14 th of August 2014, (b) the same image after processing with atmospheric correction and SPM concentrations (mg l) and (c) enlarge the area of the Pinios river estuary with the SPM values (mg/l) and the cyclonic circulation. 4. Conclusions The waters of the Thermaikos Gulf are characterized by low concentrations of suspended particulate matter (0-2.5 mg/l), without presenting specific seasonal fluctuations, constituting an attraction for the developing tourism along the coast. The distribution of suspended material is clearly influenced by river discharges. In addition, in August 2014 a well-developed cyclone eastwards the Pinios river was observed. Acknowledgments This work is part of the research project with code 11SYN Workshops on Sustainable Development and Safe Use of Greek Beaches (BEACHTOUR) financed by the European Regional Development Fund (ERDF), the European Union and National Resources. 5. Bibliography Karageorgis, A. P., Gardner, W. D., Georgopoulos, D., Mishonov, A. V., Krasakopoulou, E., Anagnostou, C Particle dynamics in the Eastern Mediterranean Sea: A synthesis based on light transmission, PMC, and POC archives ( ). Deep-Sea Research Part I: Oceanographic Research Paper, 55-2, Karageorgis, A., P. and Anagnostou, C Particulate matter spatial temporal distribution and associated surface sediment properties: Thermaikos Gulf and Sporades Basin, NW Aegean Sea. Continental Shelf Research, 21, Karageorgis, A., P., Anagnostou, C., Georgopoulos, D. and Albuisson, M Distribution of suspended particulate matter determined by in-situ observations and satellite images in the NW Aegean Sea (Greece). Geo-Marine Letters, 20, Lazogiannis, K., Paraskevopoulou, V., Poulos, S., Teou, X., Kotsopoulos, S., Farsirotou, E., Ghionis, G., Matiatos, I., Panagopoulos, A., Sifnioti, D. E., Giannouli, D. I., Tsanakas, K., Dassenakis, M., Drakopoulos, P. G., Botsou, F., Alexiou, I. & Alexopoulos, J. D Seasonal variation of water discharge and suspended sediment concentration of the Pinios River

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